KIEV, Ukraine — The political confrontation between President Viktor Yanukovich and his opposition reached a fever pitch Sunday as protesters, armed with flares, bricks and a bulldozer, were rebuffed as they attempted to storm the presidential administration building.

Demonstrators did manage to take over the City Council headquarters and two other municipal buildings on a day that saw the largest demonstrations in Ukraine since the 2004-05 Orange Revolution. The political opposition is demanding that Yanukovich reconsider his decision to reject closer ties with the European Union, which in effect realigned Ukraine with Russia.

Dozens of injuries were reported in clashes between demonstrators and police. No deaths were reported, however, and most of the protesters were peaceful.

"We are pressing a clear set of demands and want to resolve the crisis legally and peacefully," said Petro Poroshenko, a billionaire businessman and opposition lawmaker. "But the situation has a potential to get out of control here as the authorities are sending their agents to incite the crowds to attack the police."

By early afternoon, the crowd — later estimated at 300,000 — filled most of Khreshchatyk Street, Kiev's central thoroughfare, and Independence Square, where protesters listened to fiery speeches by opposition leaders. The mood shifted when a group of masked youths deploying a bulldozer attempted to break through the ranks of riot police in front of the presidential headquarters less than a mile from the rally site.

The bulldozer stopped short of hitting anyone, and the masked protesters ran away. Soon after, though, hundreds of young men, many in hoods and masks, approached the police line and began throwing flares. Police counterattacked with clubs and tear gas.

The protesters, their numbers swelling, responded by dismantling part of a nearby brick wall and throwing bricks at the police. The police, covering themselves with metal shields, refrained from using firearms, using noise grenades instead, to deafening effect.

"They call us provocateurs, but we are not!" Artyom, a 20-year-old student who wouldn't give his last name, insisted as he panted in exertion a few dozen yards from the presidential offices. "There are so many of us. My friends and I don't understand why we don't all storm the president's place and do away with the regime."

The confrontation lasted at least a couple of hours, with brief interruptions to allow ambulances to pick up the injured.

But just as the demonstration threatened to veer into hand-to-hand combat between protesters and police, Vitali Klitschko, the 6-foot-7 super heavyweight boxing champion and opposition leader, broke through the lines of attackers. Towering two heads over the crowd, he raised his arms and shouted at the top of his lungs for both sides to stop fighting.

"I am telling you, get back!" Klitschko screamed at the crowd, pushing the young men closest to him with such force that they nearly fell down. "You have no business here. Don't fall into a trap!"

With that, the tense crowd began to reluctantly back up. Other opposition leaders and their followers joined in to stop the confrontation, persuading a majority of youths to return from the presidential headquarters to nearby Independence Square.

"We are holding a peaceful demonstration and have nothing to do with these masked provocateurs," Oleg Tyagnibok, a radical opposition leader and prominent lawmaker, said in an interview. "They are hired by the authorities to provoke a bloodshed that we are doing our best to prevent."

A day before, riot police brought in from pro-Yanukovich eastern industrial regions of Ukraine swept away a peaceful protest in Independence Square by beating young men and women with clubs and boots, injuring at least 40. The authorities said the protesters were obstructing the erection of a Christmas tree in the city's main square.

Kiev's police chief, Valery Koryak, resigned Sunday after taking responsibility for the violent tactics, although the officers involved were not his.

One analyst said Yanukovich needs to make major concessions to the opposition or face the kind of uprising seen in Egypt.

"To last as president at least until 2015 [when his term expires], Yanukovich absolutely needs to sign the association agreement with the EU," Vadim Karasyov, director of the Institute of Global Strategies, a Kiev think tank, said in an interview. "In any case, tomorrow some consultations must begin in parliament on how to remedy the situation, which may give Yanukovich some little time to get his act together."

Karasyov added that "hawks" in Yanukovich's administration, who represent the security and law enforcement wing, tried to intimidate the opposition, but "Ukraine is not Belarus," referring to the neighboring post-Soviet republic led by the authoritarian Alexander Lukashenko. "They tried to lure the radical opposition into storming the presidential office today but failed. If they go on like this instead of bargaining, they will see a million people in the streets of Kiev next week."

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